Sulfate radical formation by Cr(III) activation of peroxydisulfate - Diphenylcarbazide spectrophotometric determination of sulfate radical and its scavenging activity.

Even though sulfate anion radical (SO) is a very reactive oxidant used in advanced oxidation processes, a reliably selective and simple colorimetric method for determining this radical can hardly be found. Peroxydisulfate (SO) or peroxymonosulfate (HSO) can be activated with transition metal ions to produce SO. We have discovered that Cr(III) can be an activator for persulfate, generating Cr(VI) along with SO. By measuring the emerging chromate with diphenyl carbazide (DPC) spectrophotometry at 542 nm, we could measure both the formation of SO and its scavenging with antioxidant compounds. We could also investigate a number of UV-absorbing SO scavengers which could not be measured with other UV spectrometric methods. In addition to conventional antioxidants (phenolics such as quercetin, catechin, epicatechin, caffeic acid, thiols like cysteine and N-acetyl cysteine, and ascorbid acid), nitro-aromatics (represented by 2,4,6-trinitrophenol and 2,4-dinitrophenol) used in ammunition formulations could also be measured as scavengers. The presence of scavengers caused a reduction in the amount of Cr(VI) generated, where the difference in absorbance (ΔA) of chromate - with respect to the DPC method - in the absence and presence of scavengers was a linear function of SO scavenging capacity. Ethanol and tert-butanol were tested as solvents to show the selectivity of the method for SO. The method was statistically compared to a suitably modified ABTS/persulfate assay. The efficiency order of sulfate radical scavengers was determined and ranked (Spearman's test) using both the proposed method and modified ABTS/persulfate method to reveal a moderate correlation.